This invention relates to the removal of sediments from a body of water, and particularly to the control of accumulation of sediments and removal thereof in berthing areas for ships.
Flocculated clay sediments are transported by rivers in a largely dispersed state, but upon reaching the more saline estuarine waters, they flocculate, settle and become concentrated near the bottom in a fluid-like mud layer. This mobile fluid mud layer is moved about by tidal and other bottom currents, eventually finding its way into quiet water areas, such as ship berthing areas.
Prior methods of controlling unwanted sedimentation within estuarine harbors consist of periodically removing the accumulated sediment using conventional hydraulic or mechanical dredging techniques. The latter include floating hydraulic suction dredges, hopper dredges, clamshell dredges, etc. These methods, however, are rapidly becoming uneconomical because of rising energy costs and decreasing availability of nearby dredge disposal sites. As a result, alternative sediment management methods have begun to be developed. One method which has shown particular promise has been the scour jet array. The scour jet array acts to prevent such sediment from being incorporated into the permanent bottom sediments, by periodically expelling the fluid mud layer from the protected area using the scouring action of an array of submerged, nearbottom water jets. A basic scour jet array consists of a pump, a subsurface suction intake, a distribution pipe, control valves and multiple submerged scour jets.
As flocculated sediment falls out of suspension, it concentrates near the bottom in a loose fluid-like mud layer. This layer is easily moved about by bottom currents, because it possesses little internal strength. This movement continues until it reaches a quiet water area with minimal bottom stresses. There, it begins to consolidate, rapidly increasing its internal strength and resistance to erosion. The rate of consolidation is nonlinear with respect to time, occurring very quickly at first but later at a progressively slower rate. As a result, after a period of one or two days, the newly deposited fluid mud layer has essentially been incorporated into the permanent bottom sediments. The scour jet arrays act to break this consolidation process by remobilizing the fluid mud layer near the start of the process. The jetting action creates a region of high bed stress which easily erodes the mud and transports it from the protected area.
Field tests have shown that the scour jet array is an effective sediment control system. Unfortunately, its application, at present, is relatively limited. Laboratory and field tests have shown that the power required to scour sediment increases as the 4.5 power of the scour distance. As a consequence, for conventional scour jet arrays, scour distances in excess of 100 feet are not economical. The present invention, however, consists of several improvements and techniques which extend the economical range of a scour jet array, and dramatically increase its efficiency at lesser scour distances.